The war in Iraq still casts a giant shadow over much the government does, and that is especially true of the expanded British troop deployment to Afghanistan. No surprise then that the all-party Commons defence committee expressed concern this week about several aspects of the operation, part of the Nato-led mission to steer that war-torn and drug-ravaged country to a more stable future. MPs do not oppose the despatch of 3,300 soldiers to Helmand province, a base for supporters of the deposed Taliban regime that contains significant areas of opium poppy production, but they did raise important questions. Some were specific, such as whether the troops have enough air support or whether they need more training in handling prisoners. But the general issue is defining the role of British forces in a situation the government openly acknowledges is getting more not less dangerous.

Independent experts have coined the ominous phrase "Iraqisation" to describe what is happening in Helmand and other southern provinces where Canadian and Dutch troops are based. Suicide bombings, once unknown in Afghanistan, are on the rise. Incidents are growing in frequency, intensity, sophistication and cruelty. Three US soldiers were injured yesterday in an attack on a base that is shortly to be handed over to the British as the Americans shift focus to hunting down Taliban and al-Qaida "remnants" on the Pakistani border.

The intention of ensuring that Afghanistan becomes a working democracy after its terrible years as a failed state is a laudable if ambitious one. The problem is the tension between the objective of promoting stability and security and implementing an effective counter-narcotics strategy, so central to the functioning of the weak central government in Kabul. Nato insists its personnel will not be destroying poppy crops - which provide up to 70% of the country's income - leaving that to the Afghan authorities. But it is hard to argue with the notion that the more successful the deployment is at impeding the drugs trade, the more British troops are likely to come under attack by those involved. The nexus between opium and insurgency seems frighteningly clear.

The government has to be certain what it is trying to achieve and how long it is likely to take. MPs asked the ministry of defence to be "more forthcoming" and publicly to state the criteria by which it will judge the success of the Afghan deployment. After Iraq, the British public have a right to be told how they will tell when this mission has been accomplished.